Transcript No Slide Title

2D NMR spectroscopy
• So far we have been dealing with multiple pulses but a single
dimension - that is, 1D spectra. We have seen, however, that
a multiple pulse sequence can give different spectra which
depend on the delay times we use.
• The ‘basic’ 2D spectrum would involve repeating a multiple
pulse 1D sequence with a systematic variation of the delay
time tD, and then plotting everything stacked. A very simple
example would be varying the time before acquisition:
tD1
tD2
…
tD3
…
tDn
• We now have two time domains, one that appears during
the acquisition as usual, and one that originates from the
variable delay.
2D NMR basics
• There is some renaming that we need to do to be more in
synch with the literature:
• The first perturbation of the system (pulse) will now
be called the preparation of the spin system.
• The variable tD is renamed the evolution time, t1.
• We have a mixing event, in which information from one
part of the spin system is relayed to other parts.
• Finally, we have an acquisition period (t2) as with all
1D experiments.
• Schematically, we can draw it like this:
Preparation
Evolution
t1
Mixing
Acquisition
t2
• t1 is the variable delay time, and t2 is the normal acquisition
time. We can envision having f1 and f2, for both frequencies…
• We’ll see that this format is basically the same for all 2D
pulse sequences and experiments.
A rudimentary 2D experiment
• We’ll see how it works with the backbone of what will
become the COSY pulse sequence. Think of this pulses,
were t1 is the preparation time:
90y
90x
t2
t1
• We’ll analyze it for an off-resonance (wo) singlet for a bunch
of different t1 values. Starting after the first p / 2 pulse:
z
y
90x
x
x
wo
y
z
y
90x
x
x
wo
y
The rudimentary 2D (continued)
z
y
90x
x
x
wo
y
z
y
90x
x
wo
x
y
• The second p / 2 pulse acts only on the y axis component of
the magnetization of the <xy> plane.
• The x axis component is not affected, but its amplitude will
depend on the frequency of the line.
A(t1) = Ao * cos(wo * t1 )
The rudimentary 2D (…)
• If we plot all the spectra in a stacked plot, we get:
A(t1)
t1
t1
wo
f2 (t2)
• Now, we have frequency data in one axis (f2, which came
from t2), and time domain data in the other (t1).
• Since the variation of the amplitude in the t1 domain is also
periodic, we can build a pseudo FID if we look at the points
for each of the frequencies or lines in f2.
• One thing that we are overlooking here is that during all the
pulsing and waiting and pulsing, the signal will also be
affected by T1 and T2 relaxation.
The rudimentary 2D (…)
• Now we have FIDs in t1, so we can do a second Fourier
transformation in the t1 domain (the first one was in the t2
domain), and obtain a two-dimensional spectrum:
wo
• We have a cross-peak
where the two lines
wo intercept in the 2D map,
in this case on the
diagonal.
f1
f2
• If we had a real spectrum with a lot of signals it would be a
royal mess. We look it from above, and draw it as a contour
plot - we chop all the peaks with planes at different heights.
wo
wo
f1
f2
• Each slice is color-coded
depending on the height
of the peak.
The same with some real data
• This is data from a COSY of
pulegone...
time - time
t1
t2
time - frequency
t1
f2
frequency - frequency
f1
f2
The same with some real data
• Now the contour-plot showing all the cross-peaks:
f1
f2
• OK, were the heck did all the off-diagonal peaks came from,
and what do they mean?
• I’ll do the best I can to explain it, but again, there will be
several black-box events. We really need a mathematical
description to explain COSY rigorously.
Homonuclear correlation - COSY
• COSY stands for COrrelation SpectroscopY, and for this
particular case in which we are dealing with homonuclear
couplings, homonuclear correlation spectroscopy.
• In our development of the 2D idea we considered an isolated
spin not coupled to any other spin. Obviously, this is not really
useful.
• What COSY is good for is to tell which spin is connected to
which other spin. The off-diagonal peaks are this, and they
indicate that those two peaks in the diagonal are coupled.
• With this basic idea we’ll try to see the effect of the COSY
90y - t1- 90y - t1 pulse sequence on a pair of coupled spins. If
we recall the 2 spin-system energy diagram:
ab • •
I
bb
S
J (Hz)
••
S
I
• • • • aa
ba
I
S
• We see that if we are looking at I and apply both p / 2 pulses,
(a pseudo p pulse) we will invert some of the population of
spin S, and this will have an effect on I (polarization transfer).
Homonuclear correlation (continued)
• Since the I to S or S to I polarization transfers are the
same, we’ll explain it for I to S and assume we get the same
for S to I. We first perturb I and analyze what happens to S.
• After the first p / 2, we have the two I vectors in the x axis,
one moving at wI + J / 2 and the other at wI - J / 2. The effect
of the second pulse is that it will put the components of the
magnetization aligned with y on the -z axis, which means a
partial inversion of the I populations.
• For t1 = 0, we have complete inversion of the I spins (it is a p
pulse and the signal intensity of S does not change. For all
other times we will have a change on the S intensity that
depends periodically on the resonance frequency of I.
• The variation of the population inversion for I depends on the
cosine (or sine) of its resonance frequency. Considering that
we are on-resonance with one of the lines and if t1 = 1 / 4 J:
z
y
90y
x
x
y
J/2
Homonuclear correlation (…)
• If we do it really general (nothing on-resonance), we would
come to this relationship for the change of the S signal (after
the p / 2 pulse) as a function of the I resonance frequency
and JIS coupling:
AS(t1,t2) = Ao * sin( wI * t1 ) * sin (JIS * t1 )
* sin( wS * t2 ) * sin (JIS * t2 )
• After Fourier transformation on t1 and t2 , and considering
also the I spin, we get:
wS
wI
wI
wS
f1
f2
• This is the typical pattern for a doublet in a phase-sensitive
COSY. The sines make the signals dispersive in f1 and f2.
Summary of COSY
• The 2D spectrum has cross peaks on the diagonal as well as
off the diagonal.
• Everything is doubled, because we have I to S as well as S to
I polarization transfer.
• Exactly on the diagonal we see the normal 1D spectrum. Off
the diagonal we see all connected or coupled transitions.
Next class
• Heteronuclar correlation spectroscopy (HETCOR).
• Brief discussion on the mid-term assignment.